One of the types of data packets which Ethernet switches are required to transmit are broadcast packets, i.e. packets which are to be transmitted from one of the ingress ports to all of the egress ports, except the egress port corresponding to the ingress port (“source port”) from which the broadcast packet arrived. Shared memory output queue Ethernet switches cannot sustain excessive levels of broadcast packets, because the memory requirements increase linearly with the percentage of broadcasts in a traffic stream. This means that there is a need to limit the number of broadcasts in the system.
In the case that it is identified that the number of broadcast packets is excessive, it is known to delete selected ones of the broadcast packets, e.g. selectively based on a parameter in the header of the packet defining the importance of the packet. This is referred to as “broadcast storm control” (BSC).
Conventional methods to identify excessive amounts of broadcast packet traffic operate by counting the number of broadcasts per unit time. Once this value rises above a predefined level, BSC is turned on. When the figure drops below the predetermined level (e.g. by a certain amount, so that there is a hysteresis), BSC is turned off. This method suffers from the problem that it requires a counter for explicitly counting the broadcast packets. Additionally, since the count must be worked out per unit time, a timer is required, e.g. to decrement the counter every timer interval.